Titanium has been actively utilized in fields such as the aircraft industry because of its characteristics of being light and strong. Also, because of its high corrosion resistance, titanium is increasingly being utilized in a variety of applications such as construction materials for chemical plants, thermal and nuclear power plants, and seawater desalination plants.
However, although titanium is noted for its good corrosion resistance, the high corrosion resistance was exhibited only in limited environments such as oxidizing acid (nitric acid) environments and neutral chloride environments, e.g., a sea water environment. It was not capable of exhibiting sufficient crevice corrosion resistance in high temperature chloride environments or sufficient corrosion resistance in a non-oxidizing acidic solution such as hydrochloric acid (hereinafter also collectively referred to as “corrosion resistance”).
In order to solve the above-described problem, titanium alloys formed with a platinum group metal added to titanium have been proposed, and a number of standardized products including ASTM grade 7 and ASTM grade 17 are being used in a variety of applications.
Specifically, in the chlor-alkali industry, as a material for the anode in electrolysis, titanium alloys are used for portions where crevice corrosion may occur due to the use in a chlorine containing hot concentrated brine, e.g., a 20 to 30 percent brine having a temperature of 100° C. or higher.
Also, in the nickel or lead refining industry, titanium alloys are used as a material for reaction vessels or pipes that are exposed to a slurry containing hot concentrated sulfuric acid solution at a temperature exceeding 100° C.
Furthermore, in the field of heat exchangers, titanium alloys are used, for example, in heat exchanger tubes for salt production that are exposed to a hot concentrated brine, and heat exchanger tubes for use in incinerators for heat exchange with the exhaust gas containing chlorine, nitrogen oxides, and sulfur oxides.
In the petrochemical industry, titanium alloys are used, for example, in desulfurization reactors that are exposed to crude oil, hydrogen sulfide, ammonium chloride, or the like at elevated temperatures exceeding 100° C. during petroleum refining.
As an alloy having improved corrosion resistance for the above-mentioned applications, a Ti-0.15Pd alloy (ASTM grade 7) was developed. This titanium alloy takes advantage of the phenomenon that Pd, included in the alloy, lowers the hydrogen overvoltage and thus results in maintaining the spontaneous potential within the passivation range potential. That is, deposition and buildup of Pd leached from the alloy by corrosion causes lowering of hydrogen overvoltage to thereby maintain the spontaneous potential within the passivation range potential and achieve high corrosion resistance.
However, since ASTM grade 7 having high corrosion resistance contains Pd, which is a platinum group metal and very expensive (2200 Japanese yen per gram according to the morning edition of the Nihon Keizai Shimbun dated Feb. 9, 2011), its fields of use have been limited.
In order to solve this problem, a titanium alloy having a reduced Pd content of 0.03 to 0.1% by mass (ASTM grade 17) has been proposed and put into practical use as disclosed in Patent Literature 1. Despite the reduced Pd content as compared to that of ASTM grade 7, ASTM grade 17 exhibits high crevice corrosion resistance.
Patent Literature 2 discloses a titanium alloy that is capable of being manufactured at a reduced cost while its corrosion resistance is prevented from decreasing. The titanium alloy of Patent Literature 2 contains 0.01 to 0.12% by mass in total of at least one of platinum group metals and 5% or less by mass of at least one of Al, Cr, Zr, Nb, Si, Sn and Mn. In typical applications, titanium alloys exhibit adequate properties such as corrosion resistance if Pd is present in an amount of 0.01 to 0.12% by mass. However, to meet the need for further improvement in properties in recent years, the Pd content, particularly when reduced to less than 0.05%, is not sufficient for a titanium alloy to exhibit adequate properties such as corrosion resistance. Moreover, even in typical applications, the demand for further cost savings is increasing.
Patent Literatures 3 and 4 disclose titanium alloys containing a combination of a platinum group metal, a rare earth metal, and a transition metal, as inventions belonging to different fields of art from that of the present invention. These inventions relate to an ultra high vacuum chamber and a titanium alloy for use in ultra high vacuum chambers, respectively.
In these inventions, the addition of a platinum group metal and a rare earth metal is intended to achieve the advantage of inhibiting, in ultra high vacuum environment, the diffusion and release of the gas components forming a solid solution in the material into the vacuum. These patent literatures state that the platinum group metal acts to trap hydrogen and the rare earth element acts to trap oxygen in the titanium alloy.
Furthermore, these inventions specify, as an essential element, a transition metal selected from the group consisting of Co, Fe, Cr, Ni, Mn, and Cu in addition to the platinum group metal and the rare earth metal. These patent literatures state that the transition metal acts to fix the hydrogen atoms adsorbed on the surface of the vacuum chamber by the platinum group metal. However, it is not clear whether or not the titanium alloys of Patent Literatures 3 and 4 have corrosion resistance because there are no disclosures or suggestions in this regard.
Non-Patent Literature 1 states that Pd must be present in an amount of 0.05% or more by mass to ensure the crevice corrosion resistance of a Ti—Pd alloy, and that addition of Co, Ni, or V as a third element improves the crevice corrosion resistance.
As described above, conventional art techniques are becoming less adequate to meet the need for further improvement in properties if the Pd content is below 0.05% by mass.
Furthermore, even a Ti—Pd alloy with a Pd content of 0.05% or more by mass had a problem in that when defects such as flaws occur in the surface due to the service environment, corrosion originating at the defects is likely to develop.